Exceptions

1. Chapter 15 Exceptions

2. Throwing and Catching Exceptions • When a program runs into a problem that it cannot handle, it throws an exception. • Exceptions are either caught (handled) or unhandled. • Unhandled exceptions cause the application to exit.

3. Unhandled Exceptions • When Java encounters an unhandled exception (e.g., divide by zero), the compiler provides information that can help you find the source of the error.

4. When an Exception is Thrown • Processing stops at point of exception. • Java Virtual Machine (JVM) looks for a catch block to handle the exception. • JVM unwinds the stack, or retraces its steps searching for a catch block. • If no catch block is found, the program terminates with the default handler.

5. Catching Exceptions • A try / catch block is the general approach to exception handling. • The try block contains the code that might throw an exception. • The catch block catches and handles the exception, if thrown.

6. Throwing Exceptions • Use the throw keyword when throwing the exception. • if ( divisor == 0 ) { throw new ArithmeticException(); }

7. Exception Classifications • All exceptions ultimately derive from Throwable class, which is divided into Error and Exception subclasses • Error exceptions are thrown by Java system internal errors (e.g., “out of memory”). • Error exceptions are never thrown by Java programmers.

8. Exception Class • The Exception class is divided into IOException and RunTimeException. • IOException problems are typically beyond your control (e.g., the disk is full). • RunTimeException usually indicates a programmer error (e.g., trying to write past the end of an array).

9. Custom Exceptions • Creating your own exception classes allows you to throw more specific exceptions. • Exceptions can be used polymorphically. • Multiple exception handling must be coded in order from specific to more general. • An exception class derived from a more general exception must be caught first.

10. The throws Keyword • A method can declare that it throws a certain type of exception using the throws keyword. • “I am the readFile method and I might throw an IOException.” • public void readFile(String fileName) throws IOException

11. The finally Keyword • The finally keyword works in conjunction with a try / catch block. • finally “cleans up” after an exception • Important to include finally block to limit the chance of resource leaks due to open connections or files